1. Field of the Invention
This invention relates to a conductive heater grid design that provides performance characteristics making it amenable for use in defrosting plastic and glass panels, such as windows in vehicles.
2. Related Technology
Plastic materials, such as polycarbonate (PC) and polymethylmethyacrylate (PMMA), are currently being used in the manufacturing of numerous automotive parts and components, such as B-pillars, headlamps, and sunroofs. Automotive rear window (backlight) systems represent an application for these plastic materials due to their many identified advantages, particularly in the areas of styling/design, weight savings, and safety/security. More specifically, plastic materials offer the automotive manufacturer the ability to reduce the complexity of the rear window assembly through the integration of functional components into the molded plastic system, as well as the ability to distinguish their vehicles by increasing overall design and shape complexity. Being lighter in weight than conventional glass backlight systems, their incorporation into the vehicle may facilitate both a lower center of gravity for the vehicle (and therefore better vehicle handling & safety) and improved fuel economy. Further, enhanced safety is realized, particularly in a roll-over accident because of a greater probability of the occupant or passenger being retained in a vehicle.
Although there are many advantages associated with implementing plastic windows, these windows are not without limitations that represent technical hurdles that must be addressed prior to wide-scale commercial utilization. Limitations relating to material properties include the stability of plastics during prolonged exposure to elevated temperatures and the limited ability of plastics to conduct heat. Regarding the latter, in order to be used as a rear window or backlight on a vehicle, the plastic material must be compatible with the use of a defroster or defogging system. For commercial acceptance, a plastic backlight must meet the performance criteria established for the defrosting or defogging of glass backlights.
The difference in material properties between glass and plastics becomes quite apparent when considering heat conduction. The thermal conductivity of glass (Tc=22.39×10−4 cal/cm-sec-° C.) is approximately 4-5 times greater than that exhibited by a typical plastic (e.g., Tc for polycarbonate=4.78×10−4 cal/cm-sec-° C.). Thus, a defroster or defogger (hereafter just “defroster”) designed to work effectively on a glass window may not necessarily be efficient at defrosting or defogging (hereafter just “defrosting” or “defrost”) a plastic window. The lower thermal conductivity of the plastic may limit the dissipation of heat from the heater grid lines across the surface of the plastic window. Thus, at a similar power output, a heater grid on a glass window may defrost the entire viewing area, while the same heater grid on a plastic window may only defrost those portions of the viewing area that are close to the grid lines.
A second difference between glass and plastics that must be overcome is related to the electrical conductivity exhibited by a printed heater grid. The thermal stability of glass, as demonstrated by a relatively high softening temperature (e.g., Tsoften>>1000° C.), allows for the sintering of a metallic paste on the surface of the glass window to yield a substantially inorganic frit or metallic wire. Since the softening temperature of glass is significantly greater than the glass transition temperature of a typical plastic resin (e.g., polycarbonate Tg=145° C.), a metallic paste cannot be sintered onto a plastic panel. Rather, it must be cured on the panel at a temperature lower than the Tg of the plastic resin.
A metallic paste typically consists of metallic particles dispersed in a polymeric resin that will bond to the surface of the plastic to which it is applied. The curing of the metallic paste provides a conductive polymer matrix having closely spaced metallic particles dispersed throughout a dielectric layer. The presence of the dielectric layer (e.g., polymer) between dispersed conductive particles leads to a reduction in the conductivity, or an increase in resistance, of the cured heater grid lines, as compared to dimensionally similar heater grid lines sintered onto a glass substrate. This difference in conductivity manifests itself in poor defrosting characteristics exhibited by the plastic window, as compared to the glass window.
From the above, it is seen that there is a need in the industry for a system that will effectively defrost a plastic window with performance characteristics similar to that of a conventional glass window.